What is Sony LDAC, and how does it do it?

The announcement that Sony has 'donated' LDAC to become part of Android O makes the details of the technology even more important to understand. This article, written in 2015, now has an update posted at the end, following discussions directly with Sony engineers at IFA 2016 in Berlin. To make sense of the update, we'd suggest you read the whole article, bearing in mind that Sony's product line has moved on in the last 12 months. JF

At a recent Sony AV launch, we were introduced to the various elements of Sony’s Multi-Room system, and emphasised among the system’s merits was LDAC. This was explained to us as being a Bluetooth codec, but one capable of higher quality transmission than SBC, even better than aptX. It could, Sony claimed, transmit HiRes Audio via Bluetooth.

Our reaction was a combination of delight and disbelief. As regular readers will know, we’ve been moaning that the audio quality possible via Bluetooth has barely advanced since the original profile for stereo audio, A2DP, was delivered. Subsequent advances in Bluetooth have been dominated by the drive for low power use rather than speed or audio quality. Nobody has managed to do better than the aptX codec currently owned by CSR (just purchased by Qualcomm).

Now here is Sony claiming not only to do better, but to do three times better than SBC or even aptX, while remaining within Bluetooth transmission limitations. Can it be true?

Versions, profiles and codecs
Before examining the claims and the numbers, let’s focus on the basics of Bluetooth.

There are versions, profiles, and codecs. So your device may be Bluetooth version 3.0. The music profile within that is A2DP, as it has been within all Bluetooth versions for years now. And inside the A2DP profile, different music codecs (MP3, AAC etc.) can be employed to compress the music files into bit-rates which can pass the limitations of the Bluetooth system.

In a Bluetooth system the sending device is often referred to as the SRC, and the receiver as the SNK. 'Source' to 'sink' — easy to remember.

This A2DP, the Advanced Audio Distribution Profile, has been with us nearly a decade. Before that, the only way to send audio was through a ‘voice layer’ designed for headsets and speech, and that had a throughput of only 64kbps. The rest of Bluetooth was a ‘data layer’, so to improve audio quality (and make it stereo), A2DP was designed to grab some of the additional available data bit-rate and use it for music.

There is also AVRCP (the Audio/Video Remote Control Profile), which runs alongside to allow control data – such as sending back volume and track change commands from your headphones.

How high a bit-rate could A2DP commandeer from the data pool in this way? For 48kHz sampling, the maximum available is 345kbps. For 44.1kHz sampling, it’s 328kbps. Since uncompressed CD quality runs at 1411kbps, this was clearly too slow for full CD quality without compression.

Lossy vs lossless compression
To go further, it’s important to distinguish between two types of compression — lossless and lossy. Both reduce the size of a file, and so drop its required bit-rate. The names say it all — with lossless compression there is no loss of information whatsoever, and the reconstructed file at the end is identical to the one at the beginning. Lossless compression reduces a file size by identifying patterns which can be explained quicker than by sending the data itself. It might say ‘OK, now there are 250 zeros in a row’ rather than sending zero 250 times. It's the same information in a different way, so transmission is entirely lossless.

There is a limit to how much compression can be achieved this way, and nearly all lossless systems manage about a halving of file size, depending on the material and the method used. Apple Lossless, FLAC, Ogg Vorbis — they all achieve about 50% lossless compression.

Lossy compression reduces the file size by throwing bits away. Lossy codecs look for things they think you can’t hear, or they throw away a bit of the detail and hope no-one can hear the difference. There’s no limit to how much compression can be achieved like this — you just keeping throwing more information away. A 128k MP3 is something like one thirteenth the size of its CD equivalent. Some of this may be lossless (a halving or thereabouts, as we said), but the rest is achieved by throwing stuff away. It cannot be retrieved, no matter how many MP3 retrieval circuits manufacturers put on their products. They can only guess what has gone — it’s like trying to recolour a black-and-white photocopy.

aptX
Various audio codecs can be used within the Bluetooth A2DP profile. The mandatory one is SBC (the Sub-Band Codec), which supports mono and stereo streams at various certain sampling frequencies up to 48kHz.

Other codecs are optional (including Sony’s ATRAC family) and companies can even define their own codecs, but this will require both SRC and SNK devices to have the instructions pre-installed for that codec. Hence aptX, which is widely regarded as the highest quality Bluetooth codec yet delivered, will only work if both devices are aptX-compatible - your smartphone and your Bluetooth speaker, say. AptX compatibility requires the manufacturer to pay a licence fee to be paid to aptX’s owner (CSR/Qualcomm). Hitherto Apple has only included aptX on small runs of MacBooks, but many Android devices can take advantage of aptX for streaming to wireless speakers, soundbars and so on.

We regularly see aptX publicised as “CD-quality” — but it is not, since it is limited to 328kbps under the A2DP profile, although it is often quoted as achieving 384kbps. Taking into account around 50% lossless compression, aptX still has only about half the bit-rate required to reconstruct CD quality, so “near CD quality” is a reasonable description; we find it comparable to 320kbps AAC or MP3 files.

Sony’s LDAC
Sony Australia introduced LDAC to us when demonstrating the company’s take on the wireless multiroom audio market. (The app for the system is called SongPal Link but the system itself is simply referred to as ‘Sony Multi-Room’.)

The ecosystem of products for this at launch is a good size, with three wireless speakers including one portable, plus one soundbar and one soundbase, and the SX7 micro hi-fi system. Usefully two Sony AV receivers are also compatible — the STR-DN860 and 1060 models.

The multiroom abilities include DLNA streaming at high-resolution to 24-bit/192kHz (all except the portable player). Then there’s Bluetooth, using Sony’s own LDAC audio coding technology, which Sony claimed at the launch to enable Hi-Res Audio over Bluetooth, a bit-rate of 990kbps compared with maximum Bluetooth SBC rates of 328kbps.

Ifn fact there are three levels of LDAC, operating at 330, 660 and 990kbps — for the highest bit-rate you select Quality Priority Mode.

As with aptX, LDAC can only be used where both the source and the sink device are equipped with LDAC. At present that's only the new multiroom range, plus Sony’s own Walkman, the $1599 ZX2, and LDAC-equipped headphones in the $529 MDR-1ABT. So with the ZX2 and the headphones that’s a neat combined system for wireless LDAC. Or you could stream from, say, the Multi-Room soundbar.

But how does it do this via Bluetooth? As we’ve seen above, a bit-rate of 990kbps is far higher than the specification of the whole stereo A2DP profile of Bluetooth, let alone the standard codecs within it.

Even putting that aside, we don't understand how the numbers add up. A CD-quality stream at an uncompressed 16-bit/44.1kHz has a bit-rate of 1411kbps. So LDAC’s quoted 990kbps could easily use lossless compression (with the usual roughly halving of bit-rate) to achieve full CD quality via LDAC.

But what about that claim of 24-bit/96kHz? A 24/96 file has a bit-rate higher than 4.5Mbps. To get that losslessly into 990kbps would require a lossless compression factor down to around 20%, more than double what we’ve ever seen before. Surely, then, we said to Sony Australia, it must be a lossy compression when sending hi-res audio?

We had an email conversation with Sony Australia via its PR company Hausmann. At first they confirmed to us that at 24/96, LDAC is indeed a lossy codec using approximately 4.5 times data compression — about half of which could be lossless, the rest lossy. [Via DLNA, of course, high-res audio can pass uncompressed — network speeds are far higher.]

But this was followed up quickly by another email with information from Japan.

“While we were previously told, “LDAC is a lossy codec using approximately 4.5 times data compression”, LDAC is in fact NOT a lossy codec.

“The correct answer is: LDAC is a BT codec that employs more efficient data compression than conventional BT codec like SBC, aptX up to 3 times more.

This didn’t really address our question of whether the compression is lossy or lossless, so we replied (green text):

“Apologies, of course we understand that HRA can be received by Wi-Fi. But the Sony LDAC microsite and press materials clearly show 24/96 HRA transmission, yielding 4.5Mbps output through a 990kbps Bluetooth pipe. Not Wi-Fi, Bluetooth. The website is very clear (screen grab attached).

THE QUESTION IS - WHEN DELIVERING 24/96 VIA BLUETOOTH, IS THIS LOSSY COMPRESSION OR LOSSLESS COMPRESSION?

Lossless compression means the original data can be completely and perfectly restored. Reducing a 4.6Mbps stream to 990kbps losslessly would be a huge achievement — no other codec I'm aware of has achieved much more than 50% data reduction in lossless real-time compression of music.

So there seem three possible options. Either:

a) Sony has a miracle lossless compression system with a factor of 4.5xb) Or LDAC is a lossy compression codec when sending 24-96 via Bluetooth.c) Or the website (see attached) and press materials are in error and LDAC can't send 24/96 by Bluetooth at all, only by Wi-Fi.

The third reply from Hausmann, which references Sony Australia’s knowledgeable and approachable Andrew Hughes, is as follows:

“Apologies again for the conflicting answers. This apparently is a hard one to answer which is why I’ve been getting different answers.

“I spoke with Andrew Hughes to help clarify the different responses I’ve had from regional and he said the closest answer out of what you’ve listed below is (a), but we wouldn’t use the term ‘miracle’.

To help illustrate:

Consider a scenario where you are sending music from a LDAC HRA Walkman to an LDAC HRA headphone:

- To send over the air via BT, the file is compressed to transmit, and then at the other end, the headphone reconstructs/uncompresses the file so that the result is music that is 96/24 equivalent.

- This makes it lossless, in that music quality is not lost. Lossy on the other hand compacts the file for transmission but doesn’t reconstruct it at the other end so quality is more like MP3.

- Most HRA files are 96/24. Next level up is 192/24 which in the case of our scenario, there would be loss of quality back to a 96/24 equivalent quality. LDAC cannot reproduce quality higher than 96/24.”

So that seems unequivocal — Sony is claiming a lossless compression system capable of reconstructing upwards of 4.5Mbps from a pipe carrying only 990kbps.

We don’t know how they are doing it, or how they’re getting 990kbps through the Bluetooth A2DP profile in the first place. But we’ve been moaning about the lack of advance in Bluetooth quality for years, so credit to Sony for jumping this hurdle, and we’ll stop asking awkward questions for now…

UPDATE - One year on… September 2016
We were delighted to be offered a chance to talk direct with Sony engineers during their IFA 2016 product launches, for which we travelled to Berlin as a guest of Sony Australia.

There we were able to confirm our doubts regarding LDAC’s true high-res status. It is not high-res audio, a Sony engineer informed us at the Asia-Pacific launched, “we are not claiming it is high-res audio, but that it is close to high-res audio”. He would not quite confirm the word “lossy” as opposed to lossless, but made a point of saying that Sony always includes a qualifier to the high-res claims — in the latest literature this is the word “effectively”, as in the sentence “LDAC effectively maintains the original bit depth and frequency of 24bit/96kHz, delivering a fuller, richer Hi-Res Audio enjoyment”. Completely clear, isn’t it?

As for how Sony has squeezed LDAC’s 990kbps down the A2DP’s far more restrictive pipe, the engineer told us that LDAC adds to the bit-rate of A2DP, rather than working entirely within it.

With that conversation, our questions seemed resolved — LDAC can not fully reconstruct a 24-bit 96kHz signal, so must be considered a lossy codec, which aims to deliver the closest possible version of the original high-res signal.

UPDATE 2017 - LDAC for Android 'O'
The news that Sony has 'donated' the LDAC technology to become part of Android 'O' could be of great benefit in growing the list of available smart devices that can 'send' using LDAC. But as Trusted Reviews here correctly interpreted, it's less likely that Bluetooth headphone makers other than Sony will be so keen to license LDAC in their headphones. So lots of phones may be ready to send LDAC, but only Sony headphones available to receive it - the reasoning behind Sony's 'donation' of the technology looks pretty smart!

But with Bluetooth version 5.0’s ratification and implementation under way with no inclusion of higher bit-rate audio available as standard, despite its theoretical 800% increase in data transmission capability, high-res Bluetooth will depend on just such third-party implementations as LDAC and aptX HD.

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